1. Field of the Invention
The present invention relates to a liquid crystal display device and more particularly relates to a liquid crystal display device including a fiber-filled plastic substrate.
2. Description of the Related Art
Recently, liquid crystal displays (LCDs) have found broader and broader applications. Among other things, LCDs can be used more and more extensively in mobile telecommunications apparatuses such as cell phones and personal digital assistants (PDAs) because LCDs are thin and lightweight and dissipate much less power than other display devices. In addition, in order to use LCDs even more effectively, it has become increasingly necessary to further reduce their weight and thickness and further improve their shock resistance.
To achieve these objects, some people proposed that the conventional glass substrate of LCDs be replaced with a plastic substrate. However, in order to use a plastic substrate, or a sheet of a resin material, as a substrate for an LCD effectively, there are still a number of obstacles that must be overcome.
One of the biggest problems is the high coefficient of linear thermal expansion of a plastic substrate. Specifically, the coefficient of linear thermal expansion of glass is normally a matter of several ppm/° C., whereas that of a plastic is at least several tens of ppm/° C. If the material has that a high of a coefficient of linear thermal expansion, then the size of the material varies to a significant degree according to the temperature. Accordingly, it is difficult to form TFTs or other driver components on such an easily expandable substrate because TFTs need to be patterned with high precision. Also, even if a conventional glass substrate is used as the substrate to mount the TFTs (which will be sometimes referred to herein as a “TFT substrate” simply) and if a plastic substrate is used as its counter substrate, it is still difficult to align color filters (and/or black matrix) on the counter substrate with pixel electrodes on the TFT substrate.
To make the size of a plastic substrate much less variable by reducing the coefficient of linear thermal expansion thereof, a method of making a plastic substrate of a composite material, obtained by mixing a filler with a resin matrix, was also proposed. A substrate made of such a composite material will be referred to herein as a “composite substrate”. For example, Japanese Laid-Open Publication No. 11-2812 discloses a reflective conductive substrate including a composite substrate that is formed by impregnating a piece of glass fiber fabric with a resin and curing the resin.
On the other hand, Japanese Laid-Open Publication No. 2001-133761 discloses a plastic substrate including a composite substrate that is formed by arranging linear or striped fibers in a resin so that the fibers do not contact each other. According to Japanese Laid-Open Publication No. 2001-133761, if the composite substrate is formed by filling a fiber fabric (or woven fabric) with a resin as disclosed in Japanese Laid-Open Publication No. 11-2812, then fine unevenness will be created on the surface of the substrate by the woven and overlapped portions of the fiber fabric, thus possibly deteriorating the display quality. However, Japanese Laid-Open Publication No. 2001-133761 insists that a composite substrate with a flat surface can be obtained by arranging those fibers in a resin in that manner.
However, the present inventors analyzed the optical properties of such a composite substrate, obtained by filling a resin matrix with fibers, from various angles. As a result, the present inventors discovered that some leakage of light was observed in an LCD including the conventional composite substrate. The present inventors also discovered that such leakage of light occurred due to the refractive index anisotropy of the composite substrate and depended on the positional relationship between the transmission axis (or polarization axis) of its polarizer and the composite substrate.
That is to say, even if the unevenness formed by the woven or overlapped portions of the fibers is eliminated as disclosed in Japanese Laid-Open Publication No. 2001-133761, the display quality may still deteriorate due to the refractive index anisotropy of the composite substrate. Japanese Laid-Open Publication No. 2001-133761 does mention the refractive index distribution but is silent about the refractive index anisotropy (i.e., phase difference or retardation) and its distribution. Thus, Japanese Laid-Open Publication No. 2001-133761 failed to recognize the refractive index anisotropy of the fiber-filled composite substrate.
Meanwhile, Japanese Laid-Open Publications No. 59-33428 and No. 60-78420, for example, describe preferred arrangements of the optic axis (i.e., the axis of optical anisotropy) of a plastic substrate made of a resin material only.
Specifically, Japanese Laid-Open Publication No. 59-33428 describes that if a biaxially rolled crystalline plastic substrate (film) is used, the color unevenness resulting from the birefringence of the plastic substrate can be eliminated by arranging the axis of optical anisotropy of the plastic substrate perpendicularly to the transmission axis (or polarization axis) of the polarizer.
On the other hand, Japanese Laid-Open Publication No. 60-78420 describes that the display quality can be improved by decreasing the angle defined between the optic axis and the rubbing direction to less than 45 degrees according to the magnitude of the retardation of the plastic substrate. Japanese Laid-Open Publication No. 60-78420 also describes that if the plastic substrate has a retardation of 15 nm or less, the angle between the optic axis and the rubbing direction may be defined at random.
However, the present inventors analyzed the optical anisotropy of those composite substrates in detail to discover that even when the optic axis of the plastic substrate was defined as disclosed in Japanese Laid-Open Publication No. 59-33428 or 60-78420, the leakage of light still could not be minimized and the display quality (e.g., the front contrast ratio among other things) could not be improved.